1. Field of the Invention.
The present invention relates to stabilized polyester compositions and monofilaments thereof for use in papermachine clothing and other industrial fabrics.
2. Description of the Related Art.
Monofilaments manufactured from polyethylene terephthalate (or PET) are used extensively in the production of papermachine clothing and other industrial fabrics. A paper machine typically includes three sections. In the forming section, where the cellulosic fibers are presented to a forming fabric in the form of a slurry, the fabrics are predominantly constructed from polyester monofilaments, more specifically PET. In the forming section of the papermachine the temperature rarely exceeds 60° C. and the fabric is subjected to severe wear from suction boxes used to withdraw water from the paper web, such that the fabric life is seldom over 120 days. The paper sheet is transferred from the forming section into the press section of the papermachine and at this point the solids content of the slurry is approximately 20%. Here, the paper sheet passes through a series of nip rolls or shoe presses, and due to the need for resilience, polyamides have been the material of choice. The paper sheet has about 40% solids content as it is transferred from the press section into the dryer section of the papermachine.
In the dryer section, a textile fabric holds the paper sheet against steam-heated cylinders. The temperature of the cylinder surface can exceed 120° C. and the evaporation of water from the sheet ensures that the relative humidity remains at 100%. Fabrics composed of PET are conventionally used for most dryer fabric applications. However, towards the end of the paper machine, as the solids content approaches 80 to 90%, the cooling effect of the evaporation is reduced and the temperature to which the fabric is actually exposed increases such that the PET fabric is now subjected to significant degradation. In most applications, the life of a typical PET fabric can be in excess of 12 months. However, under these extreme conditions service life is reduced significantly.
In order to extend the service life of dryer fabrics exposed to these conditions, suppliers to the industry have used an array of materials as the constituent material of the dryer fabric. Polyphenylene sulphide (or PPS) provides excellent thermal, hydrolytic and oxidative stability. However, the PPS polymer is significantly more expensive than PET. Monofilament extrusion of PPS is more problematic, leading to a higher percentage of product rejections and therefore higher production costs.
Copolyesters derived from 1,4-cyclohexane dimethanol, terephthalic acid, isophthalic acid and esters thereof, have been suggested as a cheaper alternative. U.S. Pat. No. 5,169,499 teaches the use of such copolyesters to improve the hydrolytic stability of papermachine clothing. The large cyclohexane moiety present in the polymer backbone serves to provide steric hindrance to the hydrolytic cleavage of the ester bond. However, the cyclohexane ring also serves to increase the susceptibility of such polyesters to oxidative degradation. It is generally accepted that the oxidation of polymers follows a free radical chain reaction mechanism that is initiated by abstraction of a hydrogen atom from the polymer, forming an alkyl radical. This alkyl radical can very quickly react with oxygen to form an alkyl peroxy radical, which propagates additional reactions. Each cyclohexane ring, whilst providing steric hindrance, also introduces two tertiary hydrogen atoms into the backbone of the polymer; that is two hydrogen atoms that are each bonded to a tertiary carbon. Due to the effects of electron withdrawal, the carbon hydrogen bond strength is reduced, such that the abstraction of these hydrogen atoms is much more likely at elevated temperatures. Hence, polyesters that contain this type of cyclohexane moiety are more prone to oxidative degradation than PET which has no tertiary hydrogen atoms in its polymer backbone.
The art of stabilizing polymers to oxidation at elevated temperatures is extensive. U.S. Pat. No. 5,763,512 teaches the use of a combination of a sterically hindered phenol and a specific organic phosphite for the stabilization of polyamides, polyesters or polyketones against oxidative, thermal and/or light induced degradation. Sterically hindered phenols and other organic compounds that can form resonance-stabilized radicals, are known to scavenge alkyl and alkyl peroxy radicals formed during the oxidation of a polymer, and are commonly termed primary anti-oxidants. Tri-aryl phosphites and thioester compounds react with hydroperoxide moieties formed during oxidation, and are commonly referred to as secondary anti-oxidants. Polymer Science and Engineering, Vol. 30, No. 17, page 1041 by A. Aurebach et al. cited herein, describes blends of PCT and the use of certain anti-oxidants to improve melt stability.
U.S. Pat. No. 5,981,062 attempts to improve the stability of such polyesters through blending with polyamides, more specifically the blending of polyesters based upon a polyhydric alcohol of 1,4-cyclohexane dimethanol with 5 to 20% of a polyamide, preferably PA66. The blends are shown to improve the oxidative and hydrolytic stability of monofilaments manufactured therefrom.
Polyamide 66 is known to form gels if held at elevated temperatures for an extended period of time (see Nylon handbook published by Hanser/Gardner Publications 1995, Chapter 3, Page 55). Polyesters derived from 1,4-cyclohexane dimethanol and terephthalic acid (or its esters); i.e. PCT, or polyesters derived from 1,4-cyclohexane dimethanol and terephthalic and isophthalic acid (or their esters); i.e. PCTA, have melting points of 295° C. and 285° C. respectively. These high melting points necessitate high temperature processing and melt temperatures which can be in excess of 300° C. We have seen that this will lead to some degree of gel formation with polyamide 66 and can result in some thermal degradation of PA6. Gels occur during melt processing when cross-links form between individual polymer chains. In monofilament extrusion, the presence of gels leads to diameter variation at localized sections of the filaments that are very brittle and exhibit poor mechanical properties. It is known that these filaments break under the normal loads experienced in a weaving process. Such filaments are of an unacceptable quality.
It is known to those skilled in the art that polyesters and polyamides are generally incompatible, and tend to exhibit phase separation in the melt. Phase separation induces micro-voids and various structural defects, the effect of which is to introduce weak points observed as tenacity variation along the length of the monofilament. These defects can also affect the drawing process, reducing the efficiency of monofilament manufacture.
In addition, the blends alone do not provide sufficient oxidative stability to match that of the industry standard polyester, PET, ensuring that they, and the monofilaments and textile structures derived from them, cannot be used universally. What is needed in the art is further stabilization of such polyesters, as described herein, against thermo-oxidative degradation. Textile structures formed from such monofilaments may be woven for a plurality of said filaments, or they may be constructed from helical spiral coils of the monofilaments linked together by pintle yarns, a process that is described in U.S. Pat. No. 4,423,543.